From what I've heard, Pokemon transferred from Gen 1 Virtual Console to Gen 7 via Pokemon Bank will have 3 IVs that are guaranteed to be 31 while the other 3 stats are 0-31 (similar to Wonder card Events and Legendaries like 20th Anniversary Jirachi, Celebi, etc.). For future reference, simple questions like this are better suited for the
Simple Questions/requests thread. If you're asking in regards of a specific Pokemon and are unsure if it's legal, it would be a lot easier for us to help, if you shared all the relevant details of it, preferably by using the template that's located in
the Original post of this thread.
Pokemon caught from gen 3-5 doesn't have any symbols, The Blue Pentagon is exclusive to Pokemon that was obtained in X/Y/OR/AS. So nothing of what you mentioned indicate that the Zekrom is illegal. For future reference, I suggest using the template that's shown in
the Original post of this thread to list all the relevant details of the Zekrom. To find out more exact IV spreads than "1 of its IVs are 31", you could use an IV calculator like
the one on Psypokes, it's simple to use and easy to understand (granted, it's not updated for gen 7 yet, but it does have Pokemon from gen 1-6 like Zekrom).